Will providing a filamentous substratum in the water column and shell litter on the bottom increase settlement and post-larval survival of the scallop Argopecten purpuratus?

The marked variability in the natural recruitment of Argopecten purpuratus, a common characteristic for many marine invertebrates with a pelagic larval stages, with important consequences for community functioning, is a problem for the fishery on this species. We ran experiments in the subtidal zone in Tongoy Bay, Chile, to test whether providing a filamentous settlement substratum in the water column and shell litter on the bottom would increase the settlement and post-larval survival of scallops. We placed collectors made of Netlon® 50 cm above the sand and mud bottoms, and three and a half months later there were significantly more scallop spat on the bottom under the collectors (38.5 ind m− 2), than in areas without collectors (0 ind m− 2), or in controls where collectors were installed but a bag around the collector prevented the juveniles from falling to the bottom (4.8 ind m− 2). Also, the addition of either entire or broken scallop shells to the bottom resulted in increased settlement of juveniles on the bottom (33.7 ind m− 2 with entire shells and 48.1 ind m− 2 with broken shells), compared to plots where no shell debris was added (0 ind m− 2). The 2 week survival rate of juveniles (< 3 mm shell height) added to plots covered with entire scallop shells (12.4%) and to plots covered with broken shells (15.1%) was greater than in plots where we did not add shells (3.5%). These results suggest that substrate availability explains spatial variability of recruitment for this species, while temporal variability (between years) is mainly the consequence of larval supply. The manipulation of substrates can locally increase settlement, but will not remove the temporal variability. Whereas our experiments provide useful insights into strategies for managing or establishing local scallop populations, experiments over a longer term and at a large scale are needed to further understand the community functioning in order to develop a strategy for managing this fishery resource.